1. Field of the Invention
The present invention relates to a mobile terminal such as a cellular phone or personal digital assistant (PDA) and, more particularly, to a mobile terminal having a function of detecting its own position by using a Global Positioning System (GPS) receiver.
2. Description of the Related Art
Recently, in order to detect the position of a mobile terminal user, a mobile terminal with a Global Positioning System (GPS) receiver has been developed. This type of mobile terminal generally has an autonomous position measurement mode and assisted position measurement mode. In the autonomous position measurement mode, the mobile terminal detects its own position by receiving GPS signals transmitted from a plurality of GPS satellites, and performing position measurement calculation based on only the received signals. On the other hand, in the assisted position measurement mode, the mobile terminal detects its own position by acquiring auxiliary information necessary for position measurement calculation from a position measurement server via a communication network, and receiving GPS signals from the GPS satellites and performing position measurement calculation based on the acquired auxiliary information.
Generally, the conventional mobile terminal first performs a position measurement process in the assisted position measurement mode. When the position measurement result is abnormal, the conventional mobile terminal switches to the autonomous position measurement mode to perform the position measurement process. This technique is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-228738. By using this technique, the position measurement process takes a shorter time in the assisted position measurement mode than in the autonomous position measurement mode, thereby shortening the position detection time.
However, a certain time is generally required to acquire auxiliary information from the position measurement server, in accordance with the quality of a communication network, the response state of the position measurement server, and the like. Since the conventional mobile terminal determines to switch from the assisted position measurement mode to the autonomous position measurement mode without any consideration of the time required to acquire the auxiliary information, adequate mode switching is not always achieved. To cope with such problem, conventionally, a technique has been proposed in which a timer is set, and the mobile terminal repeatedly tries to acquire the auxiliary information within the counting time of this timer in order to acquire the auxiliary information more reliably. The mobile terminal switches to the autonomous position measurement mode when the time set in the timer expires.
However, one cause which interferes with acquisition of the auxiliary information is the problem that communication with the position measurement server cannot be performed, e.g., when the mobile terminal is located outside a communication range or when the quality of a communication network is significantly low. In such case, the auxiliary information acquisition operation is repeated many times until the time set in the timer is reached although the auxiliary information cannot be acquired. Accordingly, a long time is wastefully required until the mobile terminal switches to the autonomous position measurement mode.